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According to his theory, the exact expression of gravity came down to mass and distance. The farther apart two particles are and the less massive they are, the less the gravitational force. That's Newton's law of universal gravitation in a nutshell, and it stood unchallenged for three centuries. Then, in the s, a wild-haired physicist by the name of Albert Einstein stepped in the ring and let fly with his general theory of relativity. Einstein argued that gravity was far more than just a force; it was a curve in the fourth dimension of space and time.

Given sufficient mass, an object can cause an otherwise straight beam of light to curve. Astronomers call this effect gravitational lensing , and it's one of the primary methods of detecting unobservable cosmic phenomena such as black holes. Similarly, the less gravity there is, the faster time passes, a phenomenon known as gravitational time dilation. For instance, a clock aboard an orbiting satellite advances slightly faster than a counterpart on Earth's surface. While Einstein's theory brought gravity up to speed with modern science, we still don't know everything about gravity.

Some scientists attribute gravity to hypothetical particles called gravitons , which -- in theory -- cause objects to be attracted to one another.

What is gravity? | HowStuffWorks

Finally, there's the field of quantum gravity , in which scientists attempt to reconcile general relativity with quantum theory. Quantum theory addresses how the universe works at the smallest subatomic levels. And he found that nature stubbornly refused to cooperate. By he had essentially given up, believing that a partially successful attempt — a sort of general relativity lite — was the best that nature would allow.

His theory began to solidify, and he swiftly composed four papers, one a week, during November By the last paper he had finally found the decisive equation that launched his gravitation revolution. Four years later, general relativity made Einstein himself a celebrity. If gravity curves space, he had realized early on, a light beam passing near a massive object say, the sun would be deflected from its course.

That deflection would shift the apparent position of the source of the light say, a distant star. During a solar eclipse, such a shift could be photographed and measured. The curvature of spacetime lies at the heart of general relativity. The theory predicts that anything moving through a gravitational field undisturbed by other forces will follow a curved path called a geodesic.

From any point on the equator, for instance, the shortest path to the North Pole follows a curve — the geodesic corresponding to a meridian. If two people start out on such a trek, starting some distance apart, they pursue different curved meridians to the pole but grow closer together as they travel northward.

Defining Gravity: Crash Course Kids #4.1

As it turned out, some bending of light would have been expected even with Newtonian gravity, as Johann von Soldner had calculated unknown to Einstein more than a century earlier. But Einstein predicted precisely twice as much bending as von Soldner had. Gravity deflects light just as general relativity requires. Gravitational lensing was first observed in , but Einstein had suspected its possibility in , even before his theory had been completed.

But modern astronomical explorations proved the contrary. Einstein was also ambivalent about other consequences of general relativity. In , for instance, he raised the possibility of gravitational radiation — waves rippling through spacetime after a massive body abruptly changes its motion, as when colliding with another mass.

But later Einstein changed his mind. In , he and Nathan Rosen submitted a paper arguing that such waves did not exist after all. But their paper contained a mistake. Astronomers have already detected another offspring of general relativity, black holes, throughout the cosmos. Most of that shift could be explained by the gravitational effects exerted by other planets in the solar system. But 43 seconds of arc remained unexplained. For decades, astronomers sought a solution by searching for a postulated planet, called Vulcan, nearer to the sun than Mercury.

The Science of “Romantic Gravity”

But such a planet was never found. It was the first mathematical step toward describing black holes in space. It was , two years after his special theory of relativity had rewritten textbook notions about time and motion. But in real life, objects and people move in all sorts of nonuniform ways.

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Let the air out of a balloon, for instance. Einstein wanted to extend relativity to all forms of accelerated motions. Then his happy thought in the patent office raised hope. A person falling freely accelerates toward the ground because of gravity but feels no force until impact. Therefore, Einstein realized, gravity and acceleration are two sides of a coin.

The upward thrust of an accelerating rocket ship pins the occupants to the floor just as the gravitational pull of the Earth keeps your feet on the ground. At first progress was slow. In special relativity, measures of space or time differ for different observers. But Minkowski showed that space and time combined — spacetime — yielded a mathematical description of events that all observers could agree on. One of the first predictions of general relativity to be tested involved the bending of light.

Because a massive body, such as a star, warps spacetime around it, a light beam passing nearby should be deflected from a straight-line path.

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From Earth, light from a distant star passing near the sun would be bent in such a way as to alter the apparent position of the distant star. In , astronomers photographed stars near the sun during a solar eclipse. General relativity requires twice as much bending because the light ray is bent while passing through space already curved to begin with. Establishing such coordinates requires a frame of reference, or origin point.

Different observers will choose different origins. By , Einstein realized that his goal would require abandoning Euclidean geometry. Real space, he realized, could not conform to the idealized lines and angles of the textbooks. Gravity distorted the coordinates, just as a grid of straight lines on a rubber sheet would curve if you placed a heavy cannonball on it. But Einstein did not possess the mathematical skills to cope with non-Euclidean geometry.

Fortunately, his college friend Marcel Grossmann, an accomplished mathematician, was eager to help. It had one drawback, though — it worked for some coordinate systems, but not all possible systems. Thomas William Allies.

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The Lion of Saint Mark G. Fragments of science, V. The fundamental force of attraction that all objects with mass have for each other.


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Like the electromagnetic force, gravity has effectively infinite range and obeys the inverse-square law. At the atomic level, where masses are very small, the force of gravity is negligible, but for objects that have very large masses such as planets, stars, and galaxies, gravity is a predominant force, and it plays an important role in theories of the structure of the universe.


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Gravity is believed to be mediated by the graviton, although the graviton has yet to be isolated by experiment. Gravity is weaker than the strong force, the electromagnetic force, and the weak force. Also called gravitation See more at acceleration relativity.

A Closer Look With his law of universal gravitation, Sir Isaac Newton described gravity as the mutual attraction between any two bodies in the universe. He developed an equation describing an instantaneous gravitational effect that any two objects, no matter how far apart or how small, exert on each other. These effects diminish as the distance between the objects gets larger and as the masses of the objects get smaller.

His theory explained both the trajectory of a falling apple and the motion of the planets-hitherto completely unconnected phenomena-using the same equations.